From the viewpoint of resources and reduction of environmental loads, the use of a thorium fuel has been studied. Almost all of natural thorium exists as thorium-232. Thorium-232 absorbs neutrons, and is transformed into uranium-233 through nuclear transmutation. Uranium-233 is a fissionable nuclide, and therefore can be used as a nuclear fuel.
Reserves of thorium are larger than uranium. Therefore, the use of the thorium fuel can reduce the risk of resource depletion. Furthermore, the thorium fuel generates smaller amounts of high-radiotoxicity transuranic nuclides (TRU) than the uranium fuel. As a result, it is considered that the thorium fuel is able to reduce environmental loads.
There are reports that a cycle with the use of the thorium fuel is effective in breeding of uranium-233 in light-water reactors and fast reactors, as well as in transmuting of TRU generated by a conventional uranium fuel cycle.
What is required to make effective use of the thorium fuel cycle is a reprocessing technology to refine a nuclear fuel material from a thorium fuel. Therefore, what is required is processing a metal or metal oxide and recovering separately. Moreover, as for plutonium, the use of the technology of separately recovering plutonium comes with the issue of preventing nuclear proliferation. Therefore, the process requires a high degree of nuclear proliferation resistance so as not to allow plutonium to be recovered separately.
As for the method of processing a uranium oxide fuel that has been used as a nuclear fuel of a conventional light water reactor, the following methods have been developed: a reduction method of uranium oxide, and a method of recovering uranium, plutonium and minor actinoids. As for the method of reprocessing the light water reactor fuel, as a method of reducing uranium oxide to metal, the following methods have been developed: a chemical reduction method, which uses a reducing agent, and an electrolytic reduction method.
As for the chemical reduction method, as disclosed in Japanese Patent No. 3,763,980, the following method is available: the method of using metallic lithium as a reducing agent, and making it react with uranium, plutonium, and minor actinoids in a molten salt to reduce to metals, and recovering the metals of uranium, plutonium, and minor actinoids that are produced by the reduction.
As for the electrolytic reduction method, as disclosed in Japanese Patent No. 4,089,944, the following method is available: the method of using lithium chloride, potassium chloride, and the eutectic salt of lithium chloride and potassium chloride for an electrolytic bath to carry out electrolytic reduction of a spent oxide fuel.
Moreover, as disclosed in Japanese Patent No. 3,199,937 and Japanese Patent No. 3,486,044, the following method is available: the method of carrying out electrolytic separation of a metal fuel that is obtained by reduction, or a spent metal fuel, in an electrolysis tank that stores a molten salt phase and a metallic phase to recover the metals of uranium, plutonium, and minor actinoids.
As for the method of recovering a nuclear fuel material pertaining to uranium oxide, for uranium, plutonium, and minor actinoids, the following methods are available: a method of reduction in a molten salt, and a method of recovering by electrolysis in a molten salt. For thorium oxide, no method has been established to recover a nuclear fuel material.
If a nuclear fuel material is recovered in a similar way to that for uranium oxide, reduction cannot take place in the case of a chemical reduction method that uses metallic lithium because the thorium oxide is stable. Accordingly, it is difficult to carry out electrolytic reduction of thorium oxide with the use of lithium chloride, potassium chloride, and the eutectic salt of lithium chloride and potassium chloride. Thus, the problem is that the thorium metal cannot be recovered from the thorium oxide.